1. Field of the Invention
The present invention relates to an anchor escapement for a timepiece, such as a Swiss lever escapement.
2. Description of the Related Art
Anchor escapements generally comprise fixed limitation members in the form of walls called “solid bankings” machined in the bottom plate or in a bridge, or in the form of pins fixed to the bottom plate. These fixed limitation members serve as abutments for the anchor in order to limit the amplitude of its oscillations and define two lock positions where the anchor is in abutment against a respective one of these fixed limitation members, while a tooth of the escapement wheel is itself in abutment against the locking plane of the entry or exit pallet of the anchor. These fixed limitation members also act to protect the escapement against impacts in that they prevent the anchor from moving beyond its lock positions when the watch is subjected to impacts and thus prevent the pallets of the anchor from being able to strike the escapement wheel.
However, there are anchor escapements which do not have such fixed limitation members. In this case it is a particular arrangement or shape of the toothing of the escapement wheel and/or of the pallets which fulfils the function of limiting oscillations of the anchor during normal operation of the escapement, i.e. which defines the lock positions of the anchor. Examples of such escapements are described in the documents CH 101651, CH 569997, CH 343898, DE 1162290, GB 682566 and U.S. Pat. No. 3,146,581. With the exception of that described in document CH 569997, these escapements all have the disadvantage that, when the watch is subjected to impacts, the anchor can move beyond its lock positions until the impulse beak of one of the pallets strikes the escapement wheel. By way of illustration, FIG. 1 shows the position of an anchor escapement without fixed limitation members after an impact which has displaced the anchor in the direction indicated by the arrow F1, while the anchor was in its lock position where its entry pallet was blocking the escapement wheel. It can be seen that the impulse beak of the entry pallet is in contact with the rim of the escapement wheel. In a similar manner, upon an impact having the effect of displacing the anchor in the opposite direction (FIG. 2, arrow F2), the impulse beak of the exit pallet comes into contact with the rim of the escapement wheel. Such contacts between an edge (impulse beak) and the escapement wheel can cause considerable damage to the anchor and/or the escapement wheel, in particular if these elements, or one of them, is/are produced from a fragile material such as silicon. Furthermore, the chips which can result therefrom can move into the movement of the watch, cause damage to other components and disrupt operation of the movement.
The escapement in accordance with document CH 569997 has dihedral recesses formed in the rim of the escapement wheel, which recesses are intended to receive and lock the pallets in the lock positions of the anchor. In the event of the watch being subjected to an impact, these recesses prevent the anchor from moving beyond its lock positions. In some embodiments, small clearance gaps are also provided in the escapement wheel to prevent the impulse beak of the pallets from coming into contact with the said wheel in the said lock positions. Nevertheless, this escapement has a major disadvantage in that it requires the pallets to be of the same shape and to have identical drawing angles, which prevents the efficiency of the escapement from being optimised by adapting the shapes and dimensions of the pallets.